Project Euler Problem 9
http://projecteuler.net/index.php?section=problems&id=9
合計が1000になるピタゴラス数の積
main = print $ (\(a,b,c) -> a * b * c) $ head $ filter isPythagoreanTriplet [(x,y,1000-x-y) | x <- [1..1000], y <- [1..x]] where isPythagoreanTriplet (x,y,z) = if x^2 + y^2 == z^2 then True else False